Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most types of internal combustion engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
In an effort to reduce fuel consumption, some transmissions are designed to utilize substantial amounts of energy storage in addition to liquid fuel burned in an internal combustion engine. Most commonly, the energy storage takes the form of electric batteries. The transmission diverts power to the batteries and utilizes power from the batteries using one or more reversible electric machines, such as synchronous motors or induction motors. A vehicle that uses traditional liquid fuel and also includes electrical storage is called a hybrid electric vehicle (HEV). When the vehicle includes provisions to charge the electric batteries from an external source, the vehicle is called a plug-in hybrid electric vehicle (PHEV).
One hybrid transmission configuration is a power-split hybrid. A power-split hybrid includes two electric machines. One of the electric machines is typically called the generator and the other is typically called the motor, although both are reversible electric machines. A planetary gearset distributes power from an internal combustion engine between the generator and the transmission output. The motor drives the transmission output. When the internal combustion engine is off, the motor can propel the vehicle using energy stored in the battery. During braking, the motor can converter vehicle kinetic energy to electrical energy for storage in the battery for later use. In some operating modes, the planetary gearset sends a portion of the power from the engine to the output via a mechanical power flow path and sends the remainder of the power to the generator which converts it to electrical power. The electrical power may be stored in the battery for later use, sent to the motor to supplement the power transferred via the mechanical power flow path, or some combination of the two. In other operating modes, typically associated with high vehicle speeds, the planetary gearset may draw power from the generator and send power from both the generator and the internal combustion engine to the output via the mechanical power flow path. The electrical energy to drive the generator in these modes may be drawn from the battery, from the motor, or from some combination of the two. Due to recirculation of power through the mechanical power flow path, the motor, and the generator, efficiency in these operating modes tends to be lower.